1. Technical Field
The present invention relates to an oscillation circuit, an oscillator, an electronic apparatus, a moving object, and a frequency adjustment method of the oscillator.
2. Related Art
There is an ever increasing demand for an oscillator which can change a frequency output from an external terminal in a desired range in fields such as optical communications. As such an oscillator, there are known oscillators such as an analog type voltage controlled oscillator (for example, a voltage controlled crystal oscillator (VCXO)) in which a capacitance value of a load capacitor of an oscillation circuit changes depending on a control voltage which is applied to an external terminal and thus an output frequency changes, or a digital type oscillator in which a fractional N-PLL circuit is connected to a rear stage of a simple packaged crystal oscillator (SPXO), and an output frequency changes by changing a setting of the fractional N-PLL circuit from an external terminal.
An analog type oscillator is disclosed in, for example, JP-A-2012-64915. In the oscillator disclosed in JP-A-2012-64915, a plurality of circuits are provided and are connected in parallel to each other so as to be used as a load capacitance circuit of a quartz crystal vibrator. Each of the circuits includes a variable capacitance element whose capacitance value is controlled by a voltage and a capacitor connected in series with each other. As a result, an output frequency is variably controlled.
A digital type oscillator is disclosed in, for example, JP-T-2006-526946. In the oscillator disclosed in JP-T-2006-526946, a control voltage VC is converted into a digital value by an analog-digital converter (ADC), and a division ratio of a fractional N-PLL circuit is controlled according to the digital value so that an output frequency is variably controlled.
However, in the oscillator disclosed in JP-A-2012-64915, there are problems in that, in a case where a central frequency (an output frequency when a control voltage is a central voltage in a variable range) is changed, the quartz crystal vibrator is required to be changed, electrical correction such as changing setting data cannot be performed on the central frequency, and a limitation is put on a variable range of an output frequency.
In the oscillator disclosed in JP-T-2006-526946, since an operation speed of the ADC is limited, it is difficult to change an output frequency at a high speed (to increase an upper limit of a modulation bandwidth), and if the ADC is operated at a high speed, there is a concern that digital noise or power consumption may increase.